The invention relates to a method for synchronization in a radio communication system having a plurality of mobile stations, the radio communication system being at least partly self-organizing.
The invention also relates to a mobile station in a radio communication system, the radio communication system being at least partly self-organizing, and to a radio communication system.
Communication systems are extremely important in the field of business as well as in private use. Significant efforts are being made to link cable-based communication systems to radio communication systems. The resulting hybrid systems lead to an increase in the number of available services as well as allowing greater flexibility in terms of communication. Devices that can use different systems (multi-homing) are therefore being developed.
In this context, great importance is attached to radio communication systems due to the mobility they offer to the subscribers.
In radio communication systems, information (e.g. voice, image information, video information, SMS [Short Message Service] or other data) is transmitted between a sending station and a receiving station (base station or subscriber station) via a radio interface using electromagnetic waves. In this case, the emission of the electromagnetic waves takes place using carrier frequencies that lie in the frequency band which is designated for the system concerned.
In the case of the established GSM (Global System for Mobile Communication) mobile radio system, frequencies at 900, 1800 and 1900 MHz are used. These systems principally transfer voice, facsimile and SMS short messages (Short Message Service) as well as digital data.
For future mobile radio systems that use CDMA or TD/CDMA transmission methods, e.g. UMTS (Universal Mobile Telecommunication System) or other third-generation systems, frequencies in the frequency band of approx. 2000 MHz are planned. These third-generation systems are being developed to meet the aims of worldwide radio coverage, a broad offering of data transmission services and, most importantly, flexible management of the capacity of the radio interface, which is the interface with the fewest resources in the case of radio communication systems. In the context of these radio communication systems, the flexible management of the radio interface should primarily allow a subscriber station to send and/or receive a large volume of data at high data speeds as required.
The access of stations to the shared radio resources of the transmission medium, e.g. time, frequency, throughput or space, is governed by multiple access (MA) methods in these radio communication systems.
In the case of time division multiple access methods (TDMA), each send and receive frequency band is divided into time slots, wherein one or more cyclically repeated time slots are allocated to the stations. Using TDMA, the radio resource of time is separated in a station-specific manner.
In the case of frequency division multiple access methods (FDMA), the complete frequency domain is divided into narrow-band domains, wherein one or more narrow-band frequency domains are allocated to the stations. Using FDMA, the radio resource of frequency is separated in a station-specific manner.
In the case of code division multiple access (CDMA) methods, the throughput/information which has to be transmitted is encoded in a station-specific manner by a spreading code which is formed of a multiplicity of individual so-called chips, whereby the throughput which must be transmitted is spread randomly over a wide frequency domain in accordance with a code. The spreading codes which are used by different stations within a cell/base station are mutually orthogonal or essentially orthogonal in each case, whereby a receiver recognizes the signal throughput which is intended for the receiver and suppresses other signals. Using CDMA, the radio resource of throughput is separated in a station-specific manner by spreading codes.
In the case of orthogonal frequency multiple access methods (OFDM), the data is transferred in a broadband manner, wherein the frequency band is divided into equidistant orthogonal subcarriers, such that the simultaneous phase shifting of the subcarriers covers a two-dimensional data flow in the time-frequency domain. Using OFDM, the radio resource of frequency is separated in a station-specific manner by orthogonal subcarriers. The combined data symbols which are transferred on the orthogonal subcarriers during a time unit are called OFDM symbols.
The multiple access methods can be combined. Many radio communication systems therefore use a combination of the TDMA and FDMA methods, wherein each narrow-band frequency band is divided into time slots.
For the purpose of the aforementioned UMTS mobile radio system, a distinction is made between a so-called FDD (frequency division duplex) mode and a TDD (time division duplex) mode. In particular, the TDD mode is characterized in that a shared frequency band is used for the signal transmission in both uplink (UL) direction and in downlink (DL) direction, while the FDD mode uses a different frequency band for the two transmission directions in each case.
In radio communication connections of the second and/or third generation, information can be transmitted in a circuit-switched (CS) or packet-switched (PS) manner.
The connection between the individual stations takes place via a radio communication interface (air interface). Base station and radio network controller are usually components of a base station subsystem (RNS radio network subsystem). A cellular radio communication system normally includes a plurality of base station subsystems which are connected to a core network (CN). In this case, the radio network controller of the base station subsystem is usually connected to an access facility of the core network.
In addition to these hierarchically organized cellular radio communication systems, self-organizing wireless radio communication systems—e.g. so-called ad-hoc systems—are becoming increasingly important, this applying also in the context of cellular radio communication systems.
Self-organizing radio communication systems generally also allow the direct communication between mobile terminals, and need not have a central entity which controls the access to the transmission medium.
Self-organizing radio communication systems make it possible for data packets to be exchanged directly between moving radio stations without the involvement of base stations. Consequently, an infrastructure in the form of base stations within a cellular structure is not required in such a radio network. Instead, data packets can be exchanged between moving radio stations which are within radio range of each other. In order to allow the exchange of data packets in principle, a synchronization is required between the radio stations which are usually moving. In the case of a wireless transmission via electronic waves, this means e.g. the balancing of carrier frequency (frequency synchronization) and time slot pattern (time synchronization).
Various solutions are conceivable for the synchronization in mobile radio data networks. For example, the mobile stations can have a shared reference which is transmitted e.g. via GPS. The system therefore includes globally known time information which all mobile stations can follow (e.g. VDL Mode 4, or WO 93/01576, “A Position Indicating System”). This method is disadvantageous firstly because all mobile stations must have a cost-intensive GPS receiver. Secondly, the reception of GPS signals e.g. in buildings is not always guaranteed. Other systems such as TETRA, for example, support the selection of a master which assumes the function of a ‘clock signal generator’ for the frequency domain that is assigned to it. However, such methods preclude a high granularity in relation to the time (TDMA) and/or the code (CDMA). An FDMA component is preferably used for separating the subscribers in this case. A third group of systems, such as e.g. IEEE802.11, operate without a shared time slot pattern. The mobile stations synchronize themselves by a one-shot synchronization on the basis of the data burst which is received in each case. Reservation of resources in the form of time slots for ensuring the QoS is no longer possible in this case, however.